The present invention relates to an image signal decoder for decoding a coded image signal and converting to the previous image signal of the coding, and an image signal display system for decoding a coded image signal, converting to the previous image signal of the coding, and displaying the image signal on a display device, and a liquid crystal display (hereinafter, referred to as LCD) is used as the display device.
Examples of the known apparatuses which highly efficiently code or decode image signals are based on the standards such as the ISO/IEC 13818-2 “Coding of Audio, Picture, Multimedia and Hypermedia Information” and ISO/IEC DIS 11172 “Coding of Moving Pictures and Associated Audio ISO/IEC JTC1/SC29 WG11”.
A conventional apparatus for decoding the image signal (hereinafter is referred to as image signal decoder) will be described with reference to FIG. 6. FIG. 6 is a block diagram showing the example of the conventional apparatus for decoding the image signal. In FIG. 6, the output terminal of a variable length decoding section 601 to which a coded image signal is input is connected to the input terminal of an inverse quantization section 602. The output terminal of the inverse quantization section 602 is connected to the input terminal of an inverse discrete cosine transform (hereinafter, referred to as DCT) section 603. The output terminal of the inverse DCT section 603 is connected to the input terminal of a motion compensation section 604. The output terminal of the motion compensation section 604 is connected to the input terminal of a frame buffer 605. The output terminal of the frame buffer 605 is connected to the input terminal of the motion compensation section 604. A decoded image signal is output from the output terminal of the motion compensation section 604.
Decoding operation of the conventional image signal decoder configured above will be described. The input image signal is image data highly efficiently coded based on the ISO/IEC 13818-2 (hereinafter, referred to as coded data). On the input coded data, variable length decoding is performed by the variable length decoding section 601. By the variable length decoding, the following pieces of information are extracted: motion vector information for motion compensatio; coded image signal coefficient information; time information for playback stored in the header; and header information representative of the coding mode of each frame and the like.
The coefficient information includes quantized coefficient data and quantization scale data used for the quantization. On the coefficient information, inverse quantization is performed by the inverse quantization section 602, so that the coefficient information is restored to the original DCT coefficient information which was converted into the coefficient information through quantization. On the DCT coefficient information, inverse DCT is performed by the inverse DCT section 603, so that the DCT coefficient information is converted into the original pixel value information which was converted into the DCT coefficient information through orthogonal transformation.
When the pixel value information is that of a frame on which intra-frame coding is performed (hereinafter, referred to as I frame), the pixel value information is output without undergoing the motion compensation by the motion compensation section 604. When the pixel value information is that of a frame on which forward predictive coding is performed (hereinafter, referred to as P frame) or of a frame on which bidirectional predictive coding is performed (hereinafter, referred to B frame), the pixel value information undergoes the motion compensation. That is, the converted pixel value information undergoes the motion compensation by the motion compensation section 604 by use of the motion vector information extracted by the variable length decoding section 601, and are successively output in accordance with the time information for playback. Determining the coding mode of the frame of the pixel value information, which has been, converted by the inverse DCT section 603 (whether the frame is the I frame, the P frame or the B frame) is made based on the header information.
When the pixel value information output from the motion compensation section 604 is that of the I frame or the P frame, it is temporarily stored in the frame buffer 605 so that it is used for the next motion compensation. The frame buffer 605 is capable of storing a maximum of two frames of reference data so that bidirectionally predictive-coded data can be decoded. The frame buffer 605 has a ring buffer configuration in which when the newest frame data is input, it is stored by overwriting the frame data being oldest in time with it.
In this manner, an image signal coded by a hybrid coding method that uses both intra-frame coding and interframe coding is decoded into pixel value information and is output to the display device.
A conventional image signal display system for displaying a decoded image signal on a display device will be described with reference to FIG. 7 and FIG. 8. This image signal display system is disclosed, for example, in Japanese Laid-open Patent Application No. Hei 10-11021.
FIG. 7 is a block diagram showing the configuration of the conventional image signal display system. In FIG. 7, the output terminal of a variable length decoding section 701 to which coded data is input is connected to the input terminal of an inverse quantization section 702. The output terminal of the inverse quantization section 702 is connected to the input terminal of an inverse DCT section 703. The output terminal of the inverse DCT section 703 is connected to the input terminal of a motion compensation section 704. The output terminal of the motion compensation section 704 is connected to a frame buffer 705, an image analyzation section 706 and an output signal correction section 707. The output terminal of the frame buffer 705 is connected to the input terminal of the motion compensation section 704. The output terminal of the image analyzation section 706 is connected to the input terminal of the output signal correction section 707. An output image signal of the output signal correction section 707 is input to an image display section 708.
FIG. 8 is a block diagram showing the configuration of the output signal correction section 707. In FIG. 8, the image signal and signal level distribution information described later are input to a level correction section 801. The output terminal of the level correction section 801 is connected to the input terminal of an RGB conversion section 802. The output terminal of the RGB conversion section 802 is connected to a gamma correction section 803.
Next, the operation of the conventional image signal display system will be described with reference to FIG. 7. On the input coded data, variable length decoding is performed by the variable length decoding section 701. By the variable length decoding, the following pieces of information are extracted: motion vector information for motion compensation; coded image signal coefficient information; time information for playback stored in the header; and header information representative of the coding mode of each frame and the like.
The coefficient information extracted by the variable length decoding section 701 includes quantized coefficient data and quantization scale data used for the quantization. On the coefficient information, inverse quantization is performed by the inverse quantization section 702, so that the coefficient information is restored to the original DCT coefficient information which was converted into the coefficient information through quantization.
On the DCT coefficient information restored by the inverse quantization section 702, inverse DCT is performed by the inverse DCT section 703, so that the DCT coefficient information is restored to the original pixel value information which was converted into the DCT coefficient information through orthogonal transformation. When the pixel value information converted by the inverse DCT section 703 is that of the I frame, the pixel value information is output without undergoing the motion compensation by the motion compensation section 704. When the pixel value information is that of the P frame or the B frame, the pixel value information undergoes the motion compensation by the motion compensation section 704 by use of the motion vector information extracted by the variable length decoding section 701. Then, the pixel value information is successively output in accordance with the time information extracted by the variable length decoding section 701.
The coding mode of the frame of the pixel value information converted by the inverse DCT section 703 is determined based on the header information extracted by the variable length decoding section 701.
When the pixel value information output from the motion compensation section 704 is that of the I frame or the P frame, it is temporarily stored in the frame buffer 705 so that it is used for the next motion compensation. The frame buffer 705 is capable of storing a maximum of two frames of reference data so that bidirectionally predictive-coded data can be decoded. The frame buffer 705 has a ring buffer configuration in which when the newest frame data is input, it is stored by overwriting the frame data being oldest in time with it.
The image analyzation section 706 analyzes the pixel value information output from the motion compensation section 704, and generates intra-frame signal level distribution information (e.g. information such as the maximum signal level, the minimum signal level and an average signal level). Based on the signal level distribution information output from the image analyzation section 706, the output signal correction section 707 performs output correction on the pixel value information output from the motion compensation section 704. For example, the pixel value information output from the motion compensation section 704 is input to the level correction section 801 of the output signal correction section 707. Concurrently, to the level correction section 801, the signal level distribution information is input from the image analyzation section 706. Based on the signal level distribution information, the level correction section 801 corrects (contrast correction or level correction) the pixel value information so that the maximum and the minimum levels of the pixel value information are the same as the maximum and the minimum output levels that can be displayed by the image display device, respectively.
The pixel value information (image signal) output from the level correction section 801 is converted into RGB signals by the RGB conversion section 802. On the RGB signals, input and output correction (gamma correction) responsive to characteristics of the image display device is performed by the gamma correction section 803. In this manner, the coded data is decoded into an image signal conforming to the characteristics of the display device, is output to the image display section 708 and is displayed, for example, on an LCD monitor.
Examples of conventional image signal decoders for decoding coded image signals include playback-only apparatuses such as video CD players and DVD players. Moreover, dedicated decoder boards intended for playback on personal computers, and decoder software that realizes playback processing in the form of software are known. The image signal display system uses, as the display device, a TV monitor using a TV picture tube, a monitor using a display cathode ray tube (hereinafter, referred to as CRT), an LCD monitor, or a monitor using a plasma display panel (hereinafter, referred to as PDP).
In recent years, for saving space, an image signal display system using a flat-panel LCD monitor or PDP monitor has been required. In an image signal display system using an LCD monitor as the image display device, image quality is degraded due to characteristics inherent in LCD monitors. Since LCD monitors are low in response speed, afterimages are apt to be formed when a vigorously moving picture is displayed. Moreover, since LCD monitors provide display by the dot-matrix method, interlace interference occurs when the image signal of the interlace method is displayed. In addition, since LCD monitors are low in screen illuminance, the displayed image is low in contrast and dark.